The invention described herein may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment of any royalties thereon or therefor.
The present inventions relate to methods, systems and apparatuses for performing measurement pertaining to magnetic field, more particularly to such methods, systems and apparatuses for measuring one or more components of a magnetic field over a linear region.
Ships and submarines are constructed of ferromagnetic materials which produce magnetic field signatures, making them detectable and vulnerable to magnetic influence sea mines and detectable by airborne magnetic anomaly detection (MAD) and underwater electromagnetic surveillance systems.
To reduce the magnetic field signature of ships and submarines, coils are wrapped around the ferromagnetic hull, and fields produced which reduce the vessel""s signature. In order to control the coil currents, a degaussing (DG) system must have sensors which accurately measure the signature related magnetic fields, and control algorithms to extrapolate the spatially measured field values to regions under the ship, and adjust the coil currents to minimize the signature amplitude.
It is useful to measure magnetic fields near the hull of naval ships and submarines, so that such measured magnetic fields can be used to control advanced degaussing systems. A large number of xe2x80x9cpointxe2x80x9d sensors are presently employed, but they are expensive and not capable of satisfying the need for measuring fields at all points along the circumference of a ship or submarine hull. It is important to measure these fields produced by local hull anomalies (welds, stresses, bulkheads, etc.) and material inhomogeneities at many locations, for more effective control of the ship""s degaussing system. Ideally, by measuring the surface magnetic fields all over the hull (and thereby continuously monitoring the magnetic state of a ship or submarine hull), the magnetic field signature of the ship can be adjusted and maintained at a low level using an advanced degaussing system such as the U.S. Navy""s Advanced Closed Loop Degaussing System, thereby making a ship less vulnerable to sea mine magnetic influence fuzes.
Advanced degaussing systems require accurate and spatially distributed magnetic field measurements around the ship, so that ship mathematical model algorithms can precisely control magnetic field signatures below the ship. Some of the problems associated with measuring these fields include: large spatial gradient magnetic fields; local magnetic anomalies; induced magnetic fields caused by heading changes; and, permanent magnetization changes due to pressure-induced hull stresses. Such measurements have been made using traditional fluxgate magnetometers, short baseline gradiometers, etc.
In some cases, there are large spatial magnetic field gradients, close to the hull, which are produced by local hull anomalies (e.g., welds, stresses, bulkheads, etc.) and material inhomogeneities. xe2x80x9cPointxe2x80x9d triaxial fluxgate magnetometers and gradiometers are presently used to measure these spatial gradients; however, because of these local effects, field measurements at many locations may not be useful for controlling the shipboard degaussing system.
Fluxgate magnetometers measure the magnetic field intensity using a variety of transducer cores which, normally, are considered to be small xe2x80x9cpointxe2x80x9d field sensors (typically, about one to two inches in length). More generally, fluxgate, fiber-optic and other magnetic field sensitive transducer phenomena measure the magnetic field intensity using a variety of transducer cores which are normally considered point field measurements (wherein the transducers are typically about one to two inches in length).
Pertinent background information is provided by the following papers, each of which is hereby incorporated herein by reference: Lenz, J. E., xe2x80x9cA Review of Magnetic Sensors,xe2x80x9d IEEE Proceedings, Vol. 78, No. 6, June 1990; Gordon, D. I., R. E. Brown and J. F. Haben, xe2x80x9cMethods for Measuring the Magnetic Field,xe2x80x9d IEEE Trans. Mag., Vol. Mag-8, No. 1, March 1972; Gordon, D. I. and R. E. Brown, xe2x80x9cRecent Advances in fluxgate Magnetometry,xe2x80x9d IEEE Trans. Mag., Vol. Mag-8, No. 1, March 1972; Gordon, D. I., R. H. Lundsten, R. A. Chiarodo, H. H. Helms, xe2x80x9cA Fluxgate Sensor of High Stability for Low Field Magnetometry,xe2x80x9d IEEE Transactions on Magnetics, vol. MAG-4, 1968, pp 379-401; Acuna, M. H., xe2x80x9cFluxgate Magnetometers for Outer Planets Exploration,xe2x80x9d IEEE Transactions on Magnetics, vol. MAG-10, 1974, pp 519-23.
In view of the foregoing, it is an object of the present invention to provide method, apparatus and system for measuring magnetic field distribution in a sector of interestxe2x80x94e.g., a distance, an area or a volumexe2x80x94which includes a plurality of different spatial points (i.e., discrete locations in space).
It is another object of the present invention to provide such method, apparatus and system which are more efficient than conventional methods, apparatuses and systems.
It is a further object of this invention to provide such method, apparatus and system which are more economical than conventional methods, apparatuses and systems.
It is another object of this invention to provide such method, apparatus and system which are more reliable than conventional methods, apparatuses and systems.
Another object of the present invention to provide method, apparatus and system for continuously measuring same, for use in association with a magnetic control system such as a ship degaussing system.
According to many inventive SIM embodiments, a spatially integrating transducer magnetometer measures the magnetic field components (tangential and normal) over a very long linear region, at discrete points, and integrates component field values (the sum of the field component amplitudes) over the length of the spatially integrating tranducer magnetometer""s sensor element. A typical Spatially Integrating Magnetometer (SIM) measures the magnetic field at discrete distributed points, or summation of all field components, along a xe2x80x9clinearxe2x80x9d transducer element. A typical inventive SIM: (i) measures magnetic field amplitude components over a very long linear region, at discrete points, and (ii) integrates these component field values (the sum of the field component amplitudes) over the length of the transducer element. An xe2x80x9cintegratingxe2x80x9d fluxgate transducer magnetometer measures the magnetic field amplitude component over a linear region, and xe2x80x9cintegratesxe2x80x9d the measured values to obtain the sum of the field component amplitudes over the length of the fluxgate transducer magnetometer""s sensor element. An SIM measures the magnetic field at discrete distributed points, or summation of all field components, along a xe2x80x9clinearxe2x80x9d transducer element.
Certain conventional fluxgate magnetometers implement a magnetic core having a xe2x80x9cclosedxe2x80x9d (or xe2x80x9cclosed loopxe2x80x9d) configuration; that is, the shape of the magnetic core describes a geometrically closed figure. In other words, a closed magnetic core has a closed magnetic flux path. Two closed core configurations which are known in the art are the xe2x80x9cring corexe2x80x9d and the xe2x80x9cracetrack corexe2x80x9d configurations. See, e.g., the following references, each of which is incorporated herein by reference: Pavel Ripka, xe2x80x9cReview of Fluxgate Sensors,xe2x80x9d Sensors and Actuators A, 33 (1992), pp 129-141; Pavel Ripka, xe2x80x9cRace-Track Fluxgate Sensors,xe2x80x9d Sensors and Actuators; A, 37-38 (1993), pp 417-421; Pavel Ripka, Draxler K, Kaspar P., xe2x80x9cRace-Track Fluxgate Gradiometer,xe2x80x9d Electronic Letters, 29 (1993), pp 1193-1194; Pavel Ripka, xe2x80x9cMagnetic Sensors for Industrial and Field Applications,xe2x80x9d Sensors and Actuators A, 42 (1994), nos. 1-3, pp 394-397; Pavel Ripka, F. Primdahl, I. V. Nielsen, J. R. Petersen, A. Ranta, xe2x80x9cAC Magnetic Field Measurement Using the Fluxgate,xe2x80x9d Sensors and Actuators A, 46-47 (1995), pp 307-311; Pavel Ripka, P. Kaspar, xe2x80x9cPortable Fluxgate Magnetometer,xe2x80x9d Sensors and Actuators A, 68 (1998), pp 286-289.
The National Aeronautics and Space Administration and the Naval Surface Warfare Center, White Oak (now closed) have developed ring core fluxgate magnetometers; see, e.g., the above-mentioned references Gordon, D. I. and R. E. Brown, xe2x80x9cRecent Advances in Fluxgate Magnetometry,xe2x80x9d IEEE Trans. Mag., Vol. Mag-8, No. 1, March 1972; Gordon, D. I., R. H. Lundsten, R. A. Chiarodo, H. H. Helms, xe2x80x9cA Fluxgate Sensor of High Stability for Low Field Magnetometry,xe2x80x9d IEEE Transactions on Magnetics, vol. MAG-4, 1968, pp 379-401; Acuna, M. H., xe2x80x9cFluxgate Magnetometers for Outer Planets Exploration,xe2x80x9d IEEE Transactions on Magnetics, vol. MAG-10, 1974, pp 519-23.
It is noted that the conventional ring core and racetrack core magnetometers basically accomplish the same purpose, viz., to measure the magnetic field existing at a point in space rather than along a significant distance in space. The conventional racetrack core is essentially a slightly elongated ring core, having a moderately oval shape. A conventional ring core and a conventional racetrack core can each be considered to have a length L and a diameter D. A conventional ring core has a length-to-diameter ratio L/D of approximately one. A conventional racetrack core has a length-to-diameter ratio L/D of greater than one but no greater than about four. Depending on the application, there may be advantages associated with either the ring core or racetrack core; nevertheless, until the present invention, neither magnetometer design has been considered for implementation as anything other than xe2x80x9cpointxe2x80x9d sensors.
By contrast, the present invention uniquely features an extremely elongate closed core having the associated benefit of rendering significantly more expansive magnetic field measurements than mere xe2x80x9cpointxe2x80x9d measurements. In essence, the inventive core is an exceedingly stretched ring core which approximately defines a very elongate racetrack (oval) geometry. The oblong inventive core typically comprises two approximately parallel rectilinear (e.g., approximately linear) side core sections and two curvilinear (e.g., arcuate) end sections.
In accordance with many embodiments of the present invention, a fluxgate magnetometer comprises a slender flexible magnetic core, two drive windings and a sense winding. The magnetic core is characterized by a closed magnetic flux path, a core length, a core width and a ratio of the core length to the core width of at least ten. The magnetic core has two approximately equal lengthwise core portions and two arcuate end portions. The lengthwise core portions are longitudinally contiguously joined. Each lengthwise core portion is characterized by approximately the same lengthwise core portion length, which is substantially the core length. Each drive winding is wound over one lengthwise core portion. The sense winding is wound encompassingly with respect to the combination of the two lengthwise core portions and the two drive windings. Typically, the fluxgate magnetometer is adaptable to transmitting, via the sense winding, an electrical signal which is integratively indicative of the sensed magnetic field components over the lengthwise core portion length.
The inventive fluxgate magnetometer can be thought of as a parallel-gated fluxgate magnetometer having configurational indicia both of a rod-core (two-core) fluxgate magnetometer and a ring-core magnetometer. Like a rod-core sensor, the inventive sensor includes two straight parallel core sections; like a ring-core sensor, the entire inventive core is xe2x80x9cclosed,xe2x80x9d wherein the two straight parallel core sections are connected at their respective ends by two curved xe2x80x9cend-loopxe2x80x9d core sections, thereby closing the drive flux path.
As distinguished from conventional racetrack cores, the inventive cores have a length L and a diameter D wherein the length-to-diameter ratio L/D is at least ten and more typically much greater than ten. For instance, the U.S. Navy""s prototype IFM sensor has a length-to-diameter ratio L/D of approximately thirty. According to this invention, the inventive SIM has a length-to-diameter ratio which (depending on the embodiment) potentially is ten or a hundred or a thousand or even a million. In essence, the inventive magnetic core describes an exceedingly extended ring core transducer geometry. The extremely elongate shape of the present invention""s core beneficially affords magnetometric sensing which equates to an xe2x80x9caveragingxe2x80x9d of the magnetic field over a significant distance, viz., the length L of the core. The inventive sensor, when used in the normal fashion of a parallel-gated fluxgate magnetometer, automatically operates so as to xe2x80x9cintegratexe2x80x9d the sensed values of the magnetic field aver the length L of the core.
Since the conventional racetrack core geometry has such a low L/D ratio (less than or approximately equal to four), it yields measurements which, for all practical purposes, are xe2x80x9cpointxe2x80x9d measurements; indeed, the conventional wisdom has never attributed anything other than a xe2x80x9cpointxe2x80x9d measurement to any fluxgate magnetometer. By comparison, the racetrack core geometry according to the present invention has a sufficiently high L/D ratio (greater than or approximately equal to ten) so that it yields measurements which, for all practical purposes, are true xe2x80x9cmulti-pointxe2x80x9d or xe2x80x9clinexe2x80x9d measurements. According to this invention, the magnetic field components are spatially integrated.
Thus featured by the present invention is a naturally occurring mathematically integrative function which advantageously serves to minimize the magnetic field measurement perturbations associated with anomalous portions of the region of interest, thereby more accurately assessing the magnetic field over the expanse of such region. Moreover, intrinsic to the inventive core""s high L/D ratio is the invention""s valuable potentiality for providing such core so as to be characterized by a very long core length L, practically approaching infinite length L. In other words, extremely great surface lengths and areas of entities can be integratively sensed by the present invention.
Related to (but distinguishable from) the inventive SIM is the inventive xe2x80x9cIntegrating Fluxgate Magnetometerxe2x80x9d (IFM) which is disclosed by the aforementioned U.S. nonprovisional patent application No. 09/517,558. A typical inventive Integrating Fluxgate Magnetometer (IFM) is a fluxgate magnetometer having a rigid transducer core which is configured as a long xe2x80x9crace trackxe2x80x9d in order to integrate large component gradient magnetic field near a ferromagnetic entity, e.g., a ship hull or a large piece of machinery. A typical inventive IFM: (i) measures magnetic fields over the length of its elongated transducer element (e.g., the 30 cm length of an inventive prototype tested by the U.S. Navy), and (ii) spatially integrates the component field amplitudes.
Also related to (but distinguishable from) the inventive SIM is the inventive xe2x80x9cFerromagnetic Surface Magnetic Field Sensorxe2x80x9d (FSMFS) which is disclosed by the aforementioned U.S. nonprovisional patent application No. 09/517,559. Typically, the inventive HTS uses the ferromagnetic material of the measured entity as part of the transducer core in order to determine the magnetic characteristics of the measured entity. For instance, in a typical marine application, an inventive HTS uses the ferromagnetic ship hull material am part of the transducer core to determine its magnetic characteristics.
Also related to (but distinguishable from) the inventive SIM is the inventive xe2x80x9cStanding Wave Magnetometerxe2x80x9d (SWM) which is disclosed by the aforementioned U.S. Pat. No. 6,344,743 B1. In accordance with many embodiments of the inventive SWM, a methodology is provided for determining the distribution of a magnetic field in a spatial sector. According to a typical inventive SWM, a magnetic field amplitude value is measured at each of a plurality of points in the sector, wherein the means for measuring is characterized by a length which is defined by the points. Alternating current is applied at a high frequency having an associated wavelength which corresponds to a multiple of the length. The applied alternating current is conducted so as to establish a standing wave along the length. The measured magnetic field amplitude values are processed; this processing includes performing, over the multiple of the length, Fourier analysis based on a harmonic bias function which results from the standing wave.
The inventive SIM and the inventive IFM are similar, but differ in certain respects. According to many preferred inventive SIM embodiments, the inventive SIM basically represents an extension of the inventive IFM principal, but significantly departs from the inventive IFM transducer in terms of physical characteristics (e.g., rigidity versus flexibility), configuration, dimensions and adaptability. The IFM is typically rigid. The inventive SIM is typically characterized by flexibility rather than rigidity and is typically much longer than the inventive IFM. Moreover, the inventive SIM is often characterized by greater complexity than is the inventive IFM, since the lengthy, flexible SIM can propitiously be associated with other sensors of various kinds.
The SIM according to this invention features a closed geometry magnetic core comprising a flexible (rather than rigid) core bobbin structure and an amorphous magnetic material which covers this flexible bobbin structure. Moreover, the inventive SIM features a closed magnetic core having two parallel magnetic core sections which are contiguously disposed in relation to each other. Essentially, the two parallel magnetic core sections are connected to each other at their respective ends, and are contiguously united to form a single, lengthy magnetic transducer characterized by a closed magnetic flux path.
An inventive IFM typically has an oblong-shaped bobbin core of rigid construction, wherein there is a space or separation between essentially linear sections of the bobbin core. On the other hand, an inventive SIM has a flexible construction wherein there is no space or separation between essentially linear sections of the bobbin core; that is, they are adjacent or contiguous. Like an inventive IFM, an inventive SIM is characterized by functionality of a closed core and by geometric indicia of an elongate xe2x80x9cracetrackxe2x80x9d core. Both an inventive IFM and an inventive SIM will typically be implemented so as to be at least slightly set apart (distanced) from the surface of the ferromagnetic material being sensed. A typical inventive SIM, however, is made to be much longer than a typical IFM.
In accordance with many embodiments of the present invention, fluxgate apparatus, for sensing the magnetic field of a ferromagnetic entity, comprises a flexible magnetic core, drive winding means and sense winding means. The flexible magnetic core defines a closed magnetic flux path and at least substantially describes an elongate solid shape. The magnetic core has two longitudinal linear segments and two curvilinear segments. The two longitudinal linear segments are a first longitudinal linear segment and a second longitudinal linear segment. The first longitudinal linear segment and the second longitudinal linear segment have the same length and are adjacently coupled along their length. The magnetic core has a width across the adjacently coupled first longitudinal linear segment and second longitudinal linear segment. The width is no greater than ten percent of the length. The drive winding means includes a first drive winding and a second drive winding. The first drive winding is wound on the first longitudinal linear segment. The second drive winding is wound on the second longitudinal linear segment. The sense winding means including a sense winding which is wound so an to surround both the first drive winding and the second drive winding. Typically, the fluxgate apparatus is capable of generating an output signal which is detectable through connection with the sense winding, and wherein the output signal represents the integration of magnetic field components over the length.
The inventive SIM can have practically unlimited extent for covering very large ferromagnetic areas and entities. The SIM""s magnetic core is typically made to be flexible so as to be capable of being conformally disposed along great distances of the ferromagnetic material being sensed. In effect, the SIM admits of neighboring, bordering or xe2x80x9cliningxe2x80x9d virtually anything of virtually any length. The SIM is typically embodied as a continuous narrow, flexible strip having a conjoined double-core configuration wherein the two flexible magnetic cores are longitudinally coupled so as to form a single flexible transducer. The SIM lends itself to being disposed at great distances like a tape, wire or cable.
There are numerous possible inventive applications wherein can be used, in accordance with the inventive principles of an SIM or IFM, a distributed integrating magnetic field sensor. Examples of military applications include underwater-based (e.g., as part of an underwater submarine barrier array, in naval mines, etc.) and land-based intrusion detection systems. Examples of commercial applications include geophysical prospecting (e.g., for minerals) and other physical studies. Another potential commercial application of either the inventive SIM or the inventive IFM is in the realm of traffic control (e.g., multi-lane vehicle detection). There are numerous applications of the inventive SIM wherein a large linear magnetic field sensor could be used, including underwater surveillance barriers and land based intrusion detection systems. The inventive SIM could also be used commercially for vehicle detection in traffic control systems, and in both internal and perimeter security systems. In general, the inventive SIM and the inventive IFM share many potential applications.
The xe2x80x9cSpatially Integrating Magnetometerxe2x80x9d (xe2x80x9cSIMxe2x80x9d) according to this invention accomplishes xe2x80x9cspatialxe2x80x9d integration of measured magnetic field over a great distance. The U.S. Navy considered accomplishing same, using inventive principles, by enhancing a conventional array of xe2x80x9cpointxe2x80x9d magnetometers. According to an inventively xe2x80x9cenhancedxe2x80x9d conventional approach, these measured fields would be xe2x80x9cspatiallyxe2x80x9d integrated.
A traditional spatial magnetometer array measures fields at numerous points along the hull. For instance, according to a traditional spatial magnetometer measurement array concept such as is utilized for distributed underwater surveillance sensors, xe2x80x9cpointxe2x80x9d sensors can be located in a spatial array with electrical voltages corresponding to magnetic field component values telemetered to a central processing location. Triaxial magnetometers embedded in a cable can be used, and the cable can be used to power and communicate the measured data.
Hence, in accordance with inventive principles, an inventively enhanced traditional spatial magnetometer array (such as shown in FIG. 7-1 of the below-mentioned U.S. Navy technical report NSWCCD-TR-98/011 by John F. Scarzello and Edward C. O""Keefe) could be used to measure fields at numerous points along the hull and be xe2x80x9cspatiallyxe2x80x9d integrated. The traditional spatial magnetometer measurement array concept could inventively be rendered more efficient and adapted to inventive xe2x80x9cintegrativexe2x80x9d purposes using present electronics and telemetry technology; however, its cost, size, durability and telemetry system expandability may limit its use in advanced degaussing systems. The xe2x80x9cSpatially Integrating Magnetometerxe2x80x9d (xe2x80x9cSIMxe2x80x9d) in accordance with the presents invention provides a better methodology for spatiallyxe2x80x9d integrating measured magnetic fields over great expanses and extents.
Other objects, advantages and features of this invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings,
Incorporated herein by reference in the following technical report: Scarzello, John P. and Edward C. O""Keefe, xe2x80x9cDevelopment of Shipboard Magnetic Sensors for Degaussing System Controllers,xe2x80x9d NSWCCD-TR-98/011, Jun. 30, 1998, Machinery Research and Development Directorate Research and Development Report, Naval Surface Warfare Center, Carderock Division, West Bethesda, Md. 20817-5700. See, especially, Chapter 7 of this report. This report includes 93 pages, including 43 pages of drawings.